Abstract: The invention relates to a neurosurgical auxiliary apparatus or a brain surgery adapter (150) for neurosurgical procedures carried out through a craniotomy opening, such as imaging and/or tumor resection. The brain surgery adapter (150) comprises an adapter frame (30) arranged to be clamped and sealed (20) to the edges of the craniotomy opening (6); a preferably turnable imaging plate (70) which comprises a slide space and which is arranged to be fitted in the frame and locked in position; and an ultrasonic transducer (120) which is fitted in a transducer socket (110) provided in the slide space of the imaging plate. When the adapter according to the invention is used, the ultrasonic transducer need not be supported manually. The adapter defines a closed liquid space, and it can also be provided with instruments for performing a neurosurgical operation, such as tumor resection.